It is well known that windows can be defogged by applying heat to the windows. Most modern automobiles include a rear window defogger that removes fog, frost and ice from the rear window of the automobile by applying heat to the window. Heat is normally produced by applying an electric voltage to a pair of bus bars located on opposite sides of the window. The bus bars are joined by a plurality of thin wires that extend across the window. While wire-type rear window defoggers have found widespread use in automobiles, they have not proved to be entirely satisfactory in other environments, particularly rugged environments. One such environment is boats, particularly commercial boats used in cold climates, such as Alaska.
One of the major disadvantages of wire-type window defoggers is their fragile nature. Wire-type window defoggers are supported by a thin sheet applied to the window to be defogged. As a result, the wires of such defoggers are easily broken when an object is slid across the surface of the window on which the defogger is located. Such fragility is acceptable in connection with the rear window of an automobile since objects are seldom slid across the surface of such windows. It is not acceptable in rough environments, such as on board a commercial fishing vessel.
In the past, heaters formed of a layer of indium tin oxide (ITO) on a substrate have been proposed. In addition to being proposed for use as incubator heaters (see U.S. Pat. No. 5,119,467), they have also been proposed for use as display heaters (see U.S. Pat. No. 4,952,783). Further, thin film heaters have been proposed for use in motor cycle helmet defoggers (see U.S. Pat. No. 4,584,721).
In the past, ITO heaters have not been entirely satisfactory when proposed for use in defogging relatively large surfaces, such as the windows of a fishing vessel. One of the major difficulties with ITO heaters has been the difficulty of applying power to a large area of ITO in a manner that creates uniform heating. In the past, the heat generated at different locations of an ITO layer has varied dramatically. The heat generated near the power input end of bus bars applying power to the ITO layer has been significantly greater than the heat generated at the other end of the bus bars. The temperature differential has required either increasing the power applied to the bus bars or accepting the fact that while a portion of a window may be defrosted or defogged, other portions may not be defrosted or defogged. Obviously, defrosting or defogging only a portion of a window is an unsatisfactory solution. Increasing the power to bus bars has, in the past, resulted in the overheating and destruction of the bus bars. Shorts have also created problems. Further, because, in the past, power control circuitry has been remote from the location of the ITO heat generating layer, temperature sensing and response time have also been unsatisfactory. Also, problems have been encountered in applying a substrate supporting an ITO layer to a window, particularly as an after market product.
The present invention is directed to providing a window defogging system that overcomes the foregoing disadvantages.